In modern communication systems, signals which have been received via an antenna are first of all amplified by a particularly low-noise amplifier (LNA) to a suitable level. However, the input level of a signal that is received via the antenna is not known. The input amplifiers are thus very frequently implemented with variable gain, in order in this way to amplify signals at different input levels to the same output level.
This is achieved, for example, via implementation of a low-noise input amplifier with a stepped gain, which is also referred to as an amplifier with a gain step. The use of an amplifier such as this simplifies the design of the downstream stages. However, one problem that has been found increasingly in this case is how to achieve a low capacitive component of the input impedance with low-noise amplifiers implemented on the basis of CMOS technology. With fixed predetermined source impedances such as those for GSM or WCDMA signals, the necessary matching to 100 to 200 Ω can be achieved only with great difficulty. Owing to the increasing demand for bandwidth and high gain on the input amplifiers, this matching problem is becoming even more serious.
Furthermore, modern mobile radio systems use an SAW filter in order to suppress adjacent channel power in the received signal. However, these filters have the characteristic that they can convert differentially suppressed signals to DC signal components. Owing to the high power in the adjacent channels, the additional signal components can lead to a reduction in the sensitivity of the downstream stages. It is thus expedient to provide a high level of Common mode rejection in addition in the input amplifier itself.
One possible way for doing this is transformation to a real input impedance by means of feedback with inductive components within the amplifier circuit. However, the formation of coils in this case in an integrated circuit, particularly when based on silicon as the semiconductor, occupies a very large area. This therefore also increases the production costs.
Another amplifier circuit which allows impedance transformation without any additional coils is shown in FIG. 3, which is known to the applicant. In this case, the traditional concept of a differential amplifier formed by the two field-effect transistors M1 and M2 is followed by a source follower formed from the two transistors M3 and M4. Input impedance transformation is achieved via the additional impedances Zr, which connect the output of the amplifier circuit O and Ox to the inputs In and Inx. With careful design, this amplifier circuit allows amplification with very low noise as well as suitable impedance transformation, taking into account the output resistances of the two source followers M3 and M4.
However, the source followers M3 and M4 considerably increase the power consumption. In addition, this circuit concept does not allow any additional gain and does not allow a stepped gain by means of a gain step to be provided in a simple and efficient manner, in which the loop gain and thus the input impedance as well remain constant.